This invention generally relates to improved packaging films which can be heat sealed and then easily opened when desired. More specifically, this invention relates to improved heat sealable packaging films prepared from (1) ionomers modified with polypropylene, ethylene vinyl acetate copolymer, and, optionally, polyethylene and (2) ethylene-carboxylic acid copolymers modified with polypropylene, ethylene vinyl acetate copolymer, and, optionally, polyethylene. The resulting films are heat sealable with a relatively low heat seal initiation temperature and with consistent seal strengths over a relatively broad heat sealing temperature range. The heat seal strength, which is retained over time, can be varied or adjusted to desired values by adjusting the composition of the modified ionomer and/or ethylene-carboxylic acid copolymer blends. When opened, the seal fails cohesively in a clean manner (i.e., not stringy). Since the seal whitens upon opening, the film of this invention allows for tamper evident packaging.
Heat sealable and easily opening multilayer thermoplastic films are widely used in the packaging industry, especially for food products. These films are usually formed by coextrusion or coating equipment. An external layer of a heat sealable composition is used to form the heat seal with another layer of the heat sealable composition or another substrate material. Heat sealable, easy-opening packages made from polymeric materials are known in the art and are used in many packaging applications, including food packaging.
U.S. Pat. No. 4,279,344 (Jul. 21, 1981) provides a heat sealable, easy opening packaging compositions containing 15 to 35 percent ionomer with the balance being either polypropylene or a polypropylene-polyethylene copolymer (up to 5 percent polyethylene). These compositions required relatively high heat seal temperature (i.e., about 450xc2x0 F.).
U.S. Pat. No. 4,346,196 (Aug. 24, 1982) and U.S. Pat. No. 4,469,754 (Sep. 4, 1984) provide heat seal compositions of 20 to 80 percent ionomer and 20 to 80 percent of a terpolymer (ethylene, vinyl alkanoate or ester, and ethylenically unsaturated monocarboxylic acid). U.S. Pat. No. 4,550,141 (Oct. 29, 1985) and U.S. Pat. No. 4,539,263 (Sep. 3, 1985) provide heat sealable and peelable compositions containing 80 to 93 percent ionomer and 7 to 20 percent of a propylene/xcex1-olefin copolymer. Polypropylene homopolymers did not form satisfactory heat sealable compositions when combined with ionomers.
U.S. Pat. No. 4,689,340 (Jul. 14, 1987) provides a heat sealable composition containing up to 40 percent of a first polymer with a melt flow index of less than 5 and 60 percent or more of a second polymer having a melt flow index of greater than 20. The first polymer may be an ionomer or an ethylene vinyl acetate copolymer. The second polymer is selected from the group consisting of low density polyethylene, ethylene/vinyl acetate copolymers, and modified ethylene/vinyl acetate copolymers. When the first polymer is an ethylene/vinyl acetate copolymer, the second polymer must be a low density polyethylene.
U.S. Pat. No. 4,859,514 (Aug. 22, 1989) provides a sealing system having first and second webs which are heat sealed together to form the seal. The first web is an ionomer or an ionomer-ethylene vinyl acetate copolymer blend. The second web is a blend of ethylene vinyl acetate copolymer, ethylene 1-butene copolymer, and polypropylene.
U.S. Pat. No. 5,023,121 (Jun. 11, 1991) also provides a sealing system having first and second webs which are heat sealed together to form the seal.
The first web includes a sealant layer which is a blend of polybutene and polypropylene and a third polymeric material selected from the group consisting of: (i) ethylene vinyl acetate copolymer; (ii) low density polyethylene; (iii) linear low density polyethylene; and (iv) ionomer. The second web includes a sealant layer consisting essentially of a polymeric material selected from the group consisting of ethylene vinyl acetate copolymer, low density polyethylene, linear low density polyethylene, ionomer, and mixtures thereof. An additional layer of polymeric adhesive can be placed adjacent to and bonded to the sealant layer of the first web.
U.S. Pat. No. 5,547,752 (Aug. 20, 1996) provides heat seal compositions having 15 to 50 percent of an isotactic nonelastomeric polybutylene and 50 to 85 percent ionomer. U.S. Statutory Invention Registration H1 727 (May 5, 1998) provides a heat sealable multiple layer film having an outer layer and an adjacent interior layer. The interior layer is a blend of polypropylene and a primary resin selected from the group ionomer, ethylene vinyl acetate, low density polyethylene, and linear low density polyethylene. The exterior layer is a blend of a polybutylene and a copolymer such as low density polyethylene, linear low density polyethylene, ethylene vinyl acetate, ethylene acrylic acid, or ionomer. U.S. Pat. No. 5,891,500 (Apr. 6, 1999) provides heat sealable compositions which are blends of ionomer and polybutylene.
U.S. Pat. No. 4,870,134 provides heat sealable films formed from blends of an ethylene-carboxylic acid copolymer, a butene-1 homopolymer or copolymer, and a propylene homopolymer or copolymer. U.S. Pat. No. 5,066,543 provides heat sealable films formed from blends of an ethylene-carboxylic acid copolymer, a butene-1 homopolymer or copolymer, and an ethylene homopolymer or an ethylene unsaturated ester copolymer.
There still remains a need for heat sealable, easy opening packaging systems having improved performance. Such a system would provide low heat seal initiation temperatures with consistent heat seal strengths over a relatively wide range of heat sealing temperatures. Ideally, such a system would allow for easily adjustment of the heat seal strength for specific applications and provide a heat seal strength that remains essentially unchanged over time. Such a system would also provide for an easy opening seal which opens smoothly and cleanly (i.e., not stringy) with a uniform peel force. It would also be beneficial if the packaging system could provide a built-in tamper evident feature that would allow the consumer to readily determine if the package had been previously opened. The present invention provides such an improved heat sealable, easy opening packaging system.
This invention generally relates to heat sealable films comprising ionomer resin, polypropylene, ethylene vinyl acetate copolymer, and, optionally, polyethylene and to heat sealable films comprising ethylene-carboxylic acid copolymers modified with polypropylene, ethylene vinyl acetate copolymer, and, optionally, polyethylene. More particularly, the heat sealable films of this invention are formed from blends of about 50 to about 99 percent ionomer resin, ethylene-carboxylic acid copolymers, or mixtures thereof; about 0.5 to about 25 percent polypropylene; about 0.5 to about 25 percent ethylene-vinyl acetate copolymer; and about 0 to about 25 percent polyethylene. More preferably, the heat sealable films of this invention are formed from blends of about 60 to about 95 percent ionomer resin, ethylene-carboxylic acid copolymers, or mixtures thereof; about 2.5 to about 20 percent polypropylene; about 2.5 to about 20 percent ethylene vinyl acetate copolymer; and about 0 to about 10 percent polyethylene. Preferably the polypropylene is a homopolymer. The heat sealable films of this invention are ideally suited for extrusion coating and/or coextrusion with other film materials to form heat sealable multilayer or composite films. In such multilayer films, the present heat sealable film may be heat sealed to itself or to another film material.
The heat sealable films of this invention are especially useful in food packaging applications. The heat seal strength can be varied and controlled over a relatively large range (i.e., about 0.5 to about 4 lbs/in or about 9 to about 71 g/mm). Moreover, the present films also exhibit low heat seal initiation temperatures and a relatively flat heat seal strength profile at higher heat seal temperatures. The heat seal strength remains essentially unchanged over time. Additionally, when peeled apart, the films of this invention separate cleanly and smoothly (i.e., not stringy) under a uniform peel force. Moreover, the films of this invention visibly whiten when opened and can, therefore, be used as tamper evident packaging.
One object of the present invention is to provide a heat sealable composition comprising a blend of about 50 to about 99 percent carboxylic acid-containing resin, about 0.5 to about 25 percent polypropylene, about 0.5 to about 25 percent ethylene vinyl acetate copolymer, and about 0 to about 25 percent polyethylene, wherein the carboxylic acid-containing resin is an ionomer, an ethylene-carboxylic acid copolymer, or a mixture thereof and wherein the heat sealable composition can be heat sealed to itself or another substrate to form a heat seal with a heat seal strength of about 0.5 to about 4.0 pounds per inch. Generally, heat sealable compositions containing ionomers are preferred.
Another object of this invention is to provide a flexible heat sealable multiple layer packaging film, said film comprising at least one layer of a heat sealable adhesive material and at least one layer of a polymeric packaging film, wherein the heat sealable adhesive material comprises about 50 to about 99 percent carboxylic acid-containing resin, about 0.5 to about 25 percent polypropylene, about 0.5 to about 25 percent ethylene vinyl acetate copolymer, and about 0 to about 25 percent polyethylene, wherein the carboxylic acid-containing resin is an ionomer, an ethylene-carboxylic acid copolymer, or a mixture thereof and wherein the heat sealable adhesive material can be heat sealed to itself or to a substrate to form a peelable and easy opening heat seal with a heat seal strength of about 0.5 to about 4.0 pounds per inch. The heat sealable adhesive material can be heat sealed to itself or to other flexible polymeric packaging films (including other layers of the multiple layer packaging film or other packaging films not containing a layer of the heat sealable adhesive material).
Still another object of the present invention is to provide a method of preparing a flexible heat sealable multiple layer packaging film, said method comprising:
(1) providing a homogenous blend of a heat sealable adhesive material comprising about 50 to about 99 percent carboxylic acid-containing resin, about 0.5 to about 25 percent polypropylene, about 0.5 to about 25 percent ethylene vinyl acetate copolymer, and about 0 to about 25 percent polyethylene, wherein the carboxylic acid-containing resin is an ionomer, an ethylene-carboxylic acid copolymer, or a mixture thereof; and
(2) forming a multiple layer flexible film having an exposed layer of the homogenous blend and at least one layer of other polymeric material or paper,
wherein the exposed layer of the homogenous blend can be heat sealed to form a peelable heat seal with itself or another substrate and wherein the heat seal has a heat seal strength of about 0.5 to about 4 pounds per inch.
These and other objects and advantages of the present invention will be apparent to those of ordinary skill in the art upon consideration of the present specification.